Wireless communication has rapidly evolved over the past decades. Even today, when high performance and high bandwidth wireless communication equipment is made available there is demand for even higher performance at a higher bandwidth.
In a Multi-Output (MO) system, e.g., a Multi-Input-Multi-Output (MIMO) system or a Single-Input-Multi-Output (SIMO) system, may be subject to time-varying multi-path fading, in which a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal. There are several techniques for reducing the Multi-path fading, such as time interleaving with error correction coding, implementation of frequency diversity by use of spread spectrum techniques, transmitter power control techniques, receiving antenna diversity, transmitting antenna diversity, or combinations thereof. Each technique has its own advantages and disadvantages.
Adaptive beamforming may be applied to a signal, which is transmitted via a plurality of transmit antennas, and/or to a transmission, which is received via a plurality of receive antennas. The adaptive beamforming includes applying a complex valued vector, including a plurality of weights, to the transmitted and/or received signals in order, for example, to create an equivalent directional antenna, at the direction of a desirable user.
In transmit (Tx) beamforming, the vector of weights may include a number of weights corresponding to the number of the transmit antennas. The plurality of weights may be applied to signals transmitted via the plurality of transmit antennas, respectively, in order, for example, to maximize a Signal-to-Noise-and-Interference-Ratio (SINR) at the receiver.
In receive (Rx) beamforming, the vector of weights may include a number of weights corresponding to the number of the receive antennas. The plurality of weights may be applied to signals received via the plurality of receive antennas, respectively, in order, for example, to maximize the SINR of one or more certain users.
In some Orthogonal Frequency-Division Multiplexing (OFDM) communication systems a pre-Fast-Fourier-Transform (FFT) beamforming scheme may be utilized. The pre-FFT beamforming scheme may include applying the plurality of weights to signals in the time domain, thereby creating constant weighting in the frequency domain. The pre-FFT scheme may be applicable, for example, when a coherence bandwidth of a communication channel is relatively large, such that the communication channel may be assumed nearly constant in the communication system bandwidth. Such assumption may not be applicable, for example, to multipath channels.
In other OFDM communication systems, a post-FFT beamforming scheme may be implemented. The post-FFT beamforming scheme may include applying the plurality of weights to signals in the frequency domain, such that different weights are applied in different frequency bands. The size of the frequency bands is usually determined based on the coherence bandwidth of the communication channel.